This invention relates to a control device for a direct clutch allowed to operate under certain conditions in a vehicular automatic transmission comprising an auxiliary transmission particularly having a hydraulic torque converter, a multistage gear train selectively connected to the output of the converter, the gear-to-gear speed ratio being different, and a plurality of oil hydraulic friction clutch means for actuating these working parts; and an oil hydraulic direct clutch capable of directly linking the pump and turbine impellers of the torque converter together.
The direct clutch, when actuated, operates to directly link the pump impeller of the torque converter with its turbine impeller, eliminating the slip loss caused between both the impellers, thus largely contributing to improvements in stillness and the saving of fuel during cruising. Consequently, it is desired to enlarge the range of the use of such a direct clutch as much as possible; however, the use of a direct clutch at the speed ratio over two stages poses various problems.
The first problem is that, if the speed ratio is selectively switched to another while the direct clutch is in operation, the shock produced when the speed is changed will tend to become greater than what is produced by the conventional clutch; this is inconsistent with the proposition that the direct clutch is designed to offer gentle comfortable driving.
Accordingly, the first object of the present invention is to provide a control device used to supply a buffer zone where a direct clutch is not allowed to be actuated in the boundary between two of the speed ratios at which the direct clutch should be employed, in order to prevent the speed ratios from being shifted while the direct clutch is operating.
The second problem refers to a reduction in power efficiency due to the loss of the primary function of the torque converter for amplifying torque, if the direct clutch is actuated. When the operating zone of the direct clutch is provided at the second (2ND) and third (TOP) speeds for an automatic transmission with forward three speeds, for instance, the power efficiency is reduced by the difference between the speed ratios immediately after the speed has been changed to the third one; because the output torque curve of the engine is normally low at the time of its low speed revolution, the power efficiency is particularly felt insufficient with a light load when the shifting is carried out in good time.
The second object of the present invention is therefore to solve the above problem by not letting the buffer zone have a constant width but making the zone wider with the light load, whereas the direct clutch is actuated after the speed of revolution of the engine has been rebuilt to a certain degree.
In addition, the reduction in the power efficiency is not preferred when the engine is operated in such a state that its throttle valve is almost fully opened when the vehicle is about to enter an expressway from a ramp to join the flow of other vehicles or when it has to pass another car ahead under the pressure of necessity. The power efficiency is extremely reduced in particular when a gear train in a relatively high speed stage is employed as in the case of travelling with the TOP gear. Under the special circumstances, it is advisable to release the operation of the direct clutch at the cost of stillness and the saving of fuel.
As a result, the third object of the present invention is to solve the problem of decreasing power efficiency by releasing the operation of the direct clutch whenever a high output is required.
However, although the transmission of the high output has been achieved like a case of cruising at higher speed, the stillness as well as the saving of fuel may be also required. Accordingly, the fourth object of the present invention is to meet the requirement by allowing the direct clutch to continue to operate at a vehicle speed higher than that during travelling with a gear train in the highest speed stage.
In addition, when the engine output is not required on the occasion of adjusting the distance between one vehicle and another ahead by reducing the opening degree of the throttle valve of the engine during cruising or of controlling the speed on a downhill road, the engine brake will be applied strongly, if the direct clutch is operating; this is not preferred from the fuel-saving standpoint. In order to prevent the engine brake being applied in the region where the engine turns at the particularly high speed of revolution, the direct clutch should be preferably released from its operating condition.
Upon this, the fifth object of the present invention is to prevent the fuel-saving deterioration by releasing the actuation of the direct clutch in the region where the engine brake is strongly applied, in order to maintain the same effect of the engine brake as that resulting from an automatic transmission without a direct clutch.
To control the direct clutch in such various detailed manners as mentioned above by means of oil pressure, as in the case of the conventional automatic transmission, a considerable number of control valves would be needed; besides, a space for containing them would be required, whereas the total weight would be increased. Further, because a spool valve is normally used as a control valve, a certain amount of gap for engaging purposes is still necessary for sliding the valve. The leakage loss produced by these control valves is seriously large at high temperature and the volume of the oil hydraulic pump must be so large as to compensate the loss, so that the whole or part of the improved saving of fuel, which may have been obtained from the operation of the direct clutch, could be nullfied.
Accordingly, the sixth object of the present invention is to provide the aforementioned control device with the number of control valves being as small as possible.